An expanded graphite sheet is formed by the following process: Graphite such as natural graphite, Kish graphite, or pyrolytic graphite is treated with concentrated sulfuric acid, concentrated nitric acid, concentrated sulfuric acid and potassium chlorate, concentrated sulfuric acid and potassium nitrate, a strong oxidizing agent such as a hydrogen peroxide, or a halide of bromine, aluminum chloride, or the like, to thereby form an intercalation compound. Graphite particles (acid-treated graphite material) formed in this intercalation compound are rapidly heated, i.e., are subjected to heating treatment, for instance, at high temperatures of 950° C. or more for 1 to 10 seconds to produce cracked gas. Gaps between graphite layers are expanded by its gas pressure to form expanded graphite particles, and these expanded graphite particles are subjected to compression forming or roll forming in the presence or absence of a binder, thereby forming an expanded graphite sheet. The expanded graphite sheet thus manufactured has heat resistance possessed by graphite itself and has flexibility; therefore, the expanded graphite sheet excels in formability such as bending and compression forming. Therefore, the expanded graphite sheets are used in extensive fields of, for example, gaskets, sealing, heat insulating material, cushion material, and the like.
As expanded graphite particles for forming this expanded graphite sheet, those exhibiting low expansion rates of 20 to 70 times and those exhibiting high expansion rates of 200 to 300 times are used. In the former case in which the expanded graphite particles exhibiting low expansion rates are used, since the binder is inevitably used for sheet formation, a decline in purity and a decline in physical properties result.
In contrast, in the latter case in which the expanded graphite particles exhibiting high expansion rates are used, since the sheet formation is made possible by the expanded graphite particles alone without using the binder, the purity of the expanded graphite sheet becomes high, and various physical properties become excellent.
The above-described expanded graphite sheet, particularly the expanded graphite sheet fabricated from expanded graphite particles exhibiting a high expansion rate, excels in various physical properties; however, if it is used in air in a high-temperature range exceeding 700° C. in terms of its working conditions, there is a problem in heat resistance, resulting in the oxidative wear of graphite. Hence, there is a drawback in that the rate of oxidative wear is high.
To overcome this drawback, an expanded graphite sheet has been proposed in which expanded graphite particles exhibiting a low expansion rate are used, and phosphoric acid or a phosphate is contained in the expanded graphite particles to suppress the oxidation of graphite (published in JP-B-54-30678). In this publication, it is disclosed that sheet formation is made possible by the use of phosphoric acid or a phosphate without using an adhesive. However, even if sheet formation is made possible, since the adhesive is basically not used, various physical properties as the expanded graphite sheet, particularly mechanical properties, uniformity of the sheet, and the like, are not necessarily satisfactory. In addition, although it is disclosed that its resistance to oxidation is improved, in cases where the expanded graphite sheet is exposed for long periods of time, oxidative wear is intense, and the resistance to oxidation is not necessarily satisfactory.
The present invention has been devised in view of the above-described circumstances, and its object is to provide a heat-resistant expanded graphite sheet which demonstrates the action of suppressing the oxidation of graphite even in a high-temperature range exceeding 700° C. and exhibits a high rate of oxidative wear resistance.